Frequency reference signal generating system and method for frequency synthesizers

Abstract

A system for generating a frequency reference signal comprising an oscillator, a direct digital synthesizer coupled to the oscillator and configured to receive a signal output from the oscillator, a digital to analog converter coupled to the direct digital synthesizer and configured to receive a sampled signal from the direct digital synthesizer and to convert the sampled signal to an analog waveform, and a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal.

The invention claimed is: 
     
       1. A system for generating a frequency reference signal comprising:
 a crystal oscillator; 
 a numerically controlled oscillator coupled to the crystal oscillator and configured to receive a signal output from the crystal oscillator; 
 a digital to analog converter coupled to the numerically controlled oscillator and configured to receive a sideband signal from the numerically controlled oscillator and to generate a counting sequence according to a frequency control word stored by the numerically controlled oscillator; and 
 a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal. 
 
     
     
       2. The system of  claim 1 , wherein the crystal oscillator is at 10 MHz. 
     
     
       3. The system of  claim 1 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone. 
     
     
       4. The system of  claim 1 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone. 
     
     
       5. The system of  claim 1 , wherein the frequency reference signal is at a frequency of approximately 13 MHz. 
     
     
       6. A method for generating a frequency reference signal, the method comprising:
 receiving a signal output from a crystal oscillator by a numerically controlled oscillator; 
 outputting a sideband signal from the numerically controlled oscillator; 
 receiving the sideband signal from the numerically controlled oscillator with a digital to analog converter; 
 generating a counting sequence according to a frequency control word stored by the numerically controlled oscillator with the digital to analog converter; 
 selecting an aliased output signal received from the digital to analog converter at a Nyquist zone other than a first Nyquist zone with a bandpass filter; and 
 outputting the frequency reference signal from the bandpass filter. 
 
     
     
       7. The method of  claim 6 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone. 
     
     
       8. The method of  claim 6 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone. 
     
     
       9. The method of  claim 6 , wherein the frequency reference signal is at a frequency of approximately 13 MHz. 
     
     
       10. A system for generating a frequency reference signal comprising:
 a first oscillator; 
 a second oscillator coupled to the first oscillator and configured to receive a signal output from the first oscillator; 
 a digital to analog converter coupled to the second oscillator and configured to receive a sideband signal from the second oscillator; and 
 a bandpass filter coupled to the digital to analog converter and configured to select an aliased output signal from the digital to analog converter at a Nyquist zone other than a first Nyquist zone and to output the frequency reference signal. 
 
     
     
       11. The system of  claim 10 , wherein the digital to analog converter is further configured to generate a counting sequence according to a frequency control word stored by the second oscillator. 
     
     
       12. The system of  claim 10 , wherein the first oscillator comprises a crystal oscillator. 
     
     
       13. The system of  claim 10 , wherein the second oscillator comprise a numerically controlled oscillator. 
     
     
       14. The system of  claim 12 , wherein the crystal oscillator is at a frequency of 10 MHz. 
     
     
       15. The system of  claim 13 , wherein the numerically controlled oscillator is at a frequency of 3 MHz. 
     
     
       16. The system of  claim 10 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone. 
     
     
       17. The system of  claim 10 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone. 
     
     
       18. The system of  claim 10 , wherein the frequency reference signal is at a frequency of approximately 13 MHz. 
     
     
       19. A method for generating a frequency reference signal, the method comprising:
 receiving a signal output from a first oscillator by a second oscillator; 
 outputting a sideband signal from the second oscillator; 
 receiving the sideband signal from second controlled oscillator with a digital to analog converter; 
 selecting an aliased output signal received from the digital to analog converter at a Nyquist zone other than a first Nyquist zone with a bandpass filter; and 
 outputting the frequency reference signal from the bandpass filter. 
 
     
     
       20. The method of  claim 19 , further comprising generating a counting sequence according to a frequency control word stored by the second oscillator using the digital to analog converter. 
     
     
       21. The method of  claim 19 , wherein the first oscillator comprises a crystal oscillator. 
     
     
       22. The method of  claim 19 , wherein the second oscillator comprise a numerically controlled oscillator. 
     
     
       23. The method of  claim 21 , wherein the crystal oscillator is at a frequency of 10 MHz. 
     
     
       24. The method of  claim 22 , wherein the numerically controlled oscillator is at a frequency of 3 MHz. 
     
     
       25. The method of  claim 19 , wherein the Nyquist zone selected by the bandpass filter is a second Nyquist zone. 
     
     
       26. The method of  claim 19 , wherein the Nyquist zone selected by the bandpass filter is a third Nyquist zone. 
     
     
       27. The system of  claim 19 , wherein the frequency reference signal is at a frequency of approximately 13 MHz.